Re: [Qemu-devel] [PATCH] memory: simple memory tree printer

[Jan Kiszka]

On 2011-09-14 21:24, Avi Kivity wrote:
> On 09/14/2011 09:10 PM, Jan Kiszka wrote:
>> OK, let's try again: Do we have to model hierarchy in PIO address space
>> at all? I don't think so.
> 
> 
> We do.  A device listens to addresses 0x100-0x110.  Another BAR (at 
> 0x106) clips this to 0x100-0x106.  The pci/pci bridge clips this to 
> 0x105-0x106.

OK, but clipping does not require offsets as it does not register PIO
regions at different base addresses. It's a pure internal representation
when flatting the view.

>  The host pci bridge remaps this as 
> 0x1000000105-0x1000000106 in the memory address space space.  But 
> someone configured a cpu-local region at this address, so the cpu can't 
> reach it at all.

Mapping PIO into MMIO space is special as it needs an intermediate layer
(ie. translation handlers).

Anyway, the point is that there are device models out there
(specifically PCI devices) that already use relative PIO addresses and
models (specifically ISA) that still expect absolute addresses (/wrt to
the ISA base). I believe it is better to consolidate over one model
longterm, but we need a transition phase here as well. However, I'm
unsure yet if we really need MemoryRegion::offset for that and cannot
use MemoryRegionPortio::offset. Need to look into the details.

Jan

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